it is defined for how long a terminal (for example IP phone) declares to be registered for. If the terminal does not refresh its registration on exp. it will be removed. here from the snom tepmplate
<!--# Registraion and Subscribtion timerouts/renewal #-->
<!--# The "user_expiry" field specifies the length of time (in seconds) the phone will be registered with 3CXPS. The phone will attempt to re-register with 3CXPS before this time expires. The provisioning template sets the value to 120 sec (2 minutes). Valid values are any positive integer.--> <user_expiry idx="1" perm="">120</user_expiry>
<!--# The "user_subscrption_expiry" field specifies the length of time (in seconds) the phone will be subscribed with 3CXPS for MWI and BLF notifications. The phone will attempt to re-subscribe with 3CXPS before this time expires. The provisioning template sets the value to 900 (15 minutes). Valid values are any positive integer.-->
<user_subscription_expiry idx="1" perm="">900</user_subscription_expiry>
Yes, but the extension itself may still appear green if you are registered on other devices or if you have a PUSH-enabled extension with a recent connection (expires in 1 week)
So then the 3CX holds that information of the registration? cool.
It's a Spectralink phone i'm using (8440) and it was taken from a Vertical Wave IP500 system so it probably has some old config in there.
The UAC (endpoint) sends the registration to the UAS (3CX) and defines the Expires field in seconds, so if you have a registration that doesn't appear to expire, check the logs (if available) or register it again and see what it sends on the Expires field. Maybe it defines something very high.
The phone will also probably appear in the phones list.